During operation of internal-combustion engines (hereinafter simply called internal-combustion engines,) exhaust gas is produced which contains pollutants, such as HC, CO, NOx, etc. Legal rules exist for various pollutants, such as HC, CO, NOx and particles, which fix pollutant limit values. To meet these legal requirements it is usually necessary to purify the exhaust gas that accumulates. An exhaust gas purification system is conventionally used for this purpose. Specific NOx-reducing methods based on NOx storage catalytic converters or SCR catalytic converters are increasingly being used in the case of lean internal-combustion engines, such as diesel engines and petrol lean-mix engines. SCR catalytic converters require a reducing agent for NOx reduction. Ammonia (NH3) is the reducing agent that is currently conventional. This reducing agent is currently produced by injecting a urea-water solution and hydrolyzing this in the exhaust gas system of the internal-combustion engine or “on board” the respective vehicle, and this is then supplied to the SCR catalytic converter. Provision of the reducing agent presents challenges with respect to robustness, reliability and cost-effectiveness, however. The current arrangements known from the prior art use a diaphragm pump for conveying, i.e. for transporting the reducing agent from a storage tank to the site of conversion into ammonia, the pump being suitable for the urea-water solution medium. The arrangements that are based on a backflow system have a very complex and expensive 4/2-way valve. This valve achieves pumping-out without having to reverse the pump direction. Furthermore, these arrangements require a valve to scour the pipes, and regulation of the temperature in the pipe system. Reducing agent is pumped back, moreover, i.e. counter to the feed direction during normal operation. This can take place on the one hand by using fresh air and on the other hand by way of exhaust gas through the open injector. However the latter has the drawback of the injector potentially becoming blocked up. The process of injecting the reducing agent via an injector into a generator or directly into the exhaust gas system belongs to the prior art.
A urea metering device is known from EP 1 656 986 A1. The urea metering device comprises a storage tank for the reducing agent. This is connected by a first pipe to a 2/2-way valve which introduces the reducing agent into the exhaust gas system. A pump, followed by a filter, is arranged between the storage tank and the metering valve. The pump pumps the reducing agent to the exhaust gas system, the reducing agent being metered into the exhaust gas system via actuation of the metering valve. The pump is designed in such a way that it can also convey the reducing agent from the metering valve back into the storage tank. It is therefore possible to pump reducing agent in the first pipe back into the storage tank and to completely empty the first pipe. A second pipe is also provided which can be switched via a 2/2-way valve. If during operation of the pump excess reducing agent is conveyed via the first pipe to the valve and excessive pressure is produced as a result, the valve in the second pipe is opened to reduce the pressure and a reducing agent overflow can flow back into the storage tank via the second pipe.
A metering systems is also known from DE 10 2004 054 238 A1. The metering system comprises a storage tank having a urea solution as reducing agent. The storage tank is connected by a pipe to a pump, various filters and a metering valve via which the reducing agent is metered into a metering point of an exhaust gas inlet region of a catalytic converter. A region of the feed pipe between the feed pump and the metering valve can also be emptied in the direction opposite to the feed direction during normal operation. The feed direction of the feed pump is reversed in this case and the urea solution conveyed back into the storage tank counter to the conventional feed direction during normal operation. A ventilation valve arranged upstream of the metering valve is opened and the metering valve closed.
Systems are also known from the prior art which are based on a dead headed system (no return, admission only). Such systems can be destroyed in the event of certain external conditions, such as frost, however if they do not have sufficient measures available, such as pressure-reduction or expansion possibilities for the reducing agent. Furthermore, it is not possible to reduce the pressure in the pressure pipe as desired with a diaphragm pump.
A device for metering a reducing agent by means of a metering valve system into an exhaust gas system of a motor vehicle is known from DE 199 47 197 A1. A storage tank is provided which contains the reducing agent. The storage tank is connected by a first pipe to a metering valve system. A filter element, followed by a pump mechanism, is provided in the first pipe. A pressure-reducing device is connected to the first pipe to reduce pressure from the first pipe.
A method and a device for metering a reducing agent to remove nitrogen oxides from the exhaust gas are known from DE 100 47 516 A1. The reducing agent is located in a storage tank and is conveyed via a pipe and with the aid of a pump to a metering device. A return pipe, in which a check valve is arranged, is provided parallel to the pump.